The present invention relates to a constant-current driver circuit, and, more specifically, relates to a constant-current driver circuit that drives a load element, such as an LED, by on-off controlling an output current in response to an input signal.
In a semiconductor integrated circuit device, a constant-current driver circuit or a basic operation circuit is widely used. For example, the constant-current driver circuit is mounted on an infrared-ray data communication apparatus or various portable OA devices to drive a light-emitting diode (LED) used for infrared-ray data communication. The constant-current driver circuit performs on-off control of the output current in response to a pulse transmission signal to operate the LED for emitting light or stop the emission repeatedly. Recently, with the diversification of data and the increase in the amount of communication data, a communication speedup of the constant-current driver circuit for infrared-ray data communication is required.
FIG. 1 is a schematic circuit diagram of a first constant-current driver circuit 10 of the prior art.
The constant-current driver circuit 10 includes a differential pair 11 and a constant-current source 12. The differential pair 11 includes first and second N channel MOS (NMOS) transistors Q1, Q2. The sources of the first and second transistors Q1, Q2 are connected to each other and to a low potential power supply VSS via a constant-current source 12. The drain of the first transistor Q1 is connected to a high potential power supply VDD, and the drain of the second transistor Q2 is connected to an output terminal of the constant-current driver circuit 10. A cathode of the light-emitting diode (LED) D1 is connected to the output terminal, and an anode of the light-emitting diode D1 is connected to the high potential power supply VDD.
A reference voltage Vref is provided to the gate of the first transistor Q1 (or the second transistor Q2), and a pulse input signal Sin is provided to the gate of the second transistor Q2 (or the first transistor Q1). The first and second transistors Q1, Q2 are complementarily turned on or off based on the levels of the reference voltage Vref and the input signal Sin so that an output current Iout intermittently flows into the light-emitting diode D1. As a result, the constant-current driver circuit 10 operates the light-emitting diode D1 to emit light or stop the emission in response to the input signals Sin.
The differential pair 11 of the constant-current driver circuit 10 is suitable for the high speed operation. However, a current always flows into the constant-current source 12. Thus, the current consumption of the constant-current driver circuit 10 is increased.
FIG. 2 is a schematic circuit diagram of a second constant-current driver circuit 20 of the prior art.
The constant-current driver circuit 20 includes a constant-current source 21, an analog switch 22, and a current mirror circuit 23. The current mirror circuit 23 includes input and output NMOS transistors Q3, Q4. The source of the input transistor Q3 is connected to a low potential power supply VSS, and the drain thereof is connected to a high potential power supply VDD via the analog switch 22 and the constant-current source 21. The gate of the input transistor Q3 is connected to its drain and to the gate of the output transistor Q4. The source of the output transistor Q4 is connected to a low potential power supply VSS, and the drain thereof is connected to the output terminal of the constant-current driver circuit 20. The transistors Q3 and Q4 have a size ratio of M:N therebetween. Therefore, the constant-current driver circuit amplifies the reference current Iref provided from the constant-current source 21 in accordance with the size ratio and generates the output current Iout.
FIGS. 3A and 3B are schematic circuit diagrams of a third constant-current driver circuit 30 of the prior art.
As shown in FIG. 3A, the constant-current driver circuit 30 includes a constant-current source 31, a current mirror circuit 32, and first and second analog switches 33, 34. The first and second analog switches 33, 34 are connected between the sources of the transistors Q3, Q4 of the current mirror circuit 32 and a low potential power supply VSS, respectively. As shown in FIG. 3B, the first and second analog switches 33, 34 are preferably third and fourth NMOS transistors Q5, Q6 with input signals Sin provided to their gates.
In the constant-current driver circuit 30, the third and fourth transistors Q5, Q6 are on-off controlled in synchronization with the communication signal S2. The reference current Iref provided from the constant-current source 31 is amplified in accordance with the size ratio between the first and second transistors Q3, Q4, and the output current Iout is provided to the light-emitting diode D1.
In the constant-current driver circuits 20, 30, the light-emitting diode D1 emits light or stops the emission by the on-off control of the analog switches 22, 33, and 34, and only when the light-emitting diode D1 emits light, the reference current Iref flows. Therefore, the increase in the current consumption is prevented.
A MOS transistor has the source, the drain, the gate, and a parasitic capacitance formed between a backgate substrate and the source, the drain and the gate. The value of the parasitic capacitance corresponds to the transistor size. In the constant-current driver circuit 20, the second transistor Q4 has parasitic capacitance lager than the first transistor Q3.
Thus, when the analog switch 22 is turned on by the transmission signal S2, the parasitic capacitance of the transistors Q3, Q4 is charged by the reference current Iref so that the gate voltages of the transistors Q3, Q4 increases. Therefore, time for increasing the gate voltage is determined by the parasitic capacitance, that is, the transistor size. When the switch 22 is turned off, the gate voltages of the transistors Q3, Q4 gradually decrease according to the magnitude of the parasitic capacitance. As a result, the leading edge and the trailing edge of the output current Iout of the constant-current driver circuit 20 grow dull and the high speed emission or stop of the emission of the light-emitting diode D1 becomes difficult.
When the first and second analog switches 33, 34 of the constant-current driver circuit 30 are turned on, the gate voltage Vg of the transistors Q3 or Q4 is temporarily reduced by the voltage AH as shown in FIG. 4, and then, it gradually increases according to the parasitic capacitance. As a result, the leading edge of the output current Iout grows dull, and the high speed emission or stop of the emission of the light-emitting diode D1, or the high speed switching operation, becomes difficult. To understand the effects of the parasitic capacitance more easily, FIG. 4 shows a waveform of the gate voltage Vg when the first analog switch 33 in FIG. 3A is turned on, and only the second analog switch 34 is turned on or off by the transmission signal S2.
An object of the present invention is to provide a constant-current driver circuit that on-off controls the output current at a high speed.
In a first aspect of the present invention, a constant-current driver circuit is provided. The constant-current driver circuit includes a first MOS transistor to which a reference current is provided and a second MOS transistor connected to the first MOS transistor for generating an output current having a predetermined ratio to the reference current. A switch circuit is connected to the second MOS transistor for on-off controlling the output current in accordance with the input signal. A bias circuit is connected to the gate of the second MOS transistor for providing a bias voltage to the gate of the second MOS transistor so that variation in the gate voltage of the second MOS transistor is suppressed.
In a second aspect of the present invention, a constant-current driver circuit is provided. The constant-current driver circuit includes a reference current circuit including a first MOS transistor and a first constant-current source connected to the first MOS transistor and providing a reference current to the first MOS transistor. The reference current circuit generates a gate voltage of the first MOS transistor in accordance with a first control signal. The constant-current driver circuit includes an output current circuit having a second MOS transistor which generates an output current having a predetermined ratio to the reference current. The gate of the second MOS transistor is connected to the gate of the first MOS transistor. The output current circuit includes a first switch circuit connected to the second MOS transistor in series for on-off controlling the output current in accordance with a second control signal. A bias circuit is connected to the gate of the second MOS transistor for providing a bias voltage to the gate of the second MOS transistor in accordance with a third control signal so that variation in the gate voltage of the second MOS transistor in the on control of the output current is suppressed.
In a third aspect of the present invention, a method of controlling a gate voltage of an output transistor in a constant-current driver circuit is provided. The constant-current driver includes an input transistor for receiving a reference current, an output transistor for generating an output current having a predetermined ratio to the reference current, and a switch circuit for on-off controlling the output current in response to an input signal. The method includes: providing a bias voltage to the gate of the output transistor when the output current is off controlled by the switch circuit; and generating an output current of the output transistor by on controlling the output current by the switch circuit.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.